1. Field of the Invention
The present invention relates to a press jacket for a press device in the treatment of a web material, e.g., draining (dewatering) or smoothing a fibrous pulp web, and to a press roll including the press jacket. The press jacket may include a plurality of holes formed in the surface of the press jacket.
2. Discussion of Background Information
Press jackets similar in general to the type described above may be utilized, e.g., in extended nip presses or smoothing mechanisms in the manufacturing of paper. The press jacket is conventionally formed as a continuous tube and is rotatable around its longitudinal axis via both jacket ends. The interior of the press jacket, i.e., in the axial direction, lies close to a press shoe positioned within the press jacket, i.e., to extend over the entire length of the press jacket and to extend in a circumferential direction over a pre-determined width. A pressure directed radially outwardly via a cylinder-piston arrangement may be exerted on the press shoe against the interior of the roll jacket. In this manner, a portion of the press jacket adjacent the press shoe, i.e., forming of the press nip or press zone, can be pressed against a mating roll positioned parallel to the press roll.
The fibrous pulp web to be treated is guided through the press zone, i.e., between the press roll and the mating roll, so that the pressure exerted in the press zone creates a smoothing and/or draining of the fibrous pulp web. Depending on the specific use, a felt belt may be positioned on one or both sides of the fibrous pulp web to guide the fibrous pulp web through the press zone. In this manner, the liquid removed from the fibrous pulp web may be absorbed by the felt belt(s) and transported away from the fibrous pulp web.
To improve the removal of liquid, it is conventional to form a plurality of holes in the surface of the press jacket. These holes are filled by the liquid removed from the fibrous pulp web or felt belt within the press zone. As the press roll rotates, the holes filled with liquid are emptied outside of the press zone. In this manner, upon reentering the press zone, the holes positioned in the press zone are ready to absorb liquid.
However, press jackets having such holes are problematic, particularly depending on the size and geometric arrangement of the holes in the press jacket surface. A shadow hole marking appears on the paper web being treated if the pressing and the fluid removal does not occur evenly over the surface of the paper web. Sometimes the marking is so subtle that they are not visible on the actual paper web, but become visible during the printing of the paper web.
Since the drainage holes in the roll jacket surface are usually distributed as evenly as possible over the surface of the press jacket, enabling even and sufficient draining, the hole shadow markings have a conspicuous regularity. It is this conspicuousness that is particularly troublesome. Further, variation in hole size or uneven distribution of the holes over the jacket surface is not suitable for reducing of the hole shadow markings, because the holes must have optimized dimensions and be substantially equidistant from each other to provide the necessary dewatering of the web.